Steam Turbine Bypass System

Good day everyone... in a coal fired power plant which employed a reheat steam turbine associated to it is a turbine bypass system. my questions are, 1. what is the function of the High Pressure Bypass Valve? if it is controlling the pressure of the main steam inlet for HP blades, how it does? 2. same question to number 1 but it refers to Low Pressure Bypass valve from Reheated Steam bypassing to IP/LP blades of the turbine directly to condenser? Basically, my query is all about the function of Turbine Bypass System. Thanks ahead..